Therapeutic drugs are increasingly effective in the treatment of lymphocytic leukemias, but drug induced remissions of myeloid and myelomonocytic forms of this disease are followed by relapses and death. It is proposed to extend the investigation of the potential of differentiating agents, vitamin D3 and its analogs, as adjuncts to the cytotoxic treatment of leukemia with arabinocytosine (Ara-C). This study is conducted in vitro on established cell lines, their variants resistant to Ara-C and differentiation agents, and on cells freshly isolated from leukemic patients or from normal volunteers. The action of the analogs of vitamin D on the various intermediate steps in the pathways that signal differentiation and cessation of proliferation will be examined. This will include the expression of the immediate-early genes, protein kinase C activity, phosphorylation/dephosphorylation of proteins, the DNA binding of transcription factors, and control of histone synthesis. Secondly, the mechanisms responsible for the potentiation of Ara-C cytotoxicity by vitamin D will be studied in leukemic cells susceptible to this regimen. We will determine the potential usefulness of differentiation therapy of the analogs of vitamin D which recently have become available, and have lower calcium mobilizing properties than the previously studied compounds. This will include a systematic study of molecular changes in leukemic cells subjected to sequential treatments with Ara-C and vitamin D analogs. Inhibition of cell proliferation, terminal differentiation and cytotoxicity will be related to the altered expression of proto-oncogenes and other regulatory genes. Third, the recently discovered cross-resistance of HL60 cells resistant to Ara-C to vitamin D analogs, and collateral sensitivity to etoposide, will be studied in relation to the mobilization of intracellular calcium and programmed cell death. This knowledge will be utilized to propose new therapeutic strategies for improved treatment of leukemia.